2.2 Measuring Earthquakes

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Vocabulary

Focus: The focus is the point beneath the surface where rock breaks because it is under stress and then triggers an earthquake.
Epicenter: The point that is exactly above the focus is called the epicenter.
Seismic waves: These waves are vibrations that travel through the earth at various speeds carrying the energy that is released by an earthquake.
P waves: These waves are earthquake waves that compress and also expand the surface like an accordion.
S waves: These waves are earthquakes waves which vibrate from side-to-side as well as up-and-down making a lot of damage are called S waves.
Surface waves: These waves move more slowly than P waves and S waves but surface waves create the most severe ground movement.
Seismograph: A seismograph is an instrument that records the ground movements that are caused by seismic waves as they travel through the Earth.
Magnitude: The strength of an earthquake.
Mercalli scale: This is a scale developed to rate earthquakes by their intensity.
Richter scale: This is a scale developed to rate the size of the seismic waves.
Moment magnitude scale: This is a rating system used to estimate the quantity of the energy that is released by an earthquake.

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Outline

Introduction

• Every day there are at least 8,000 earthquakes occurring everywhere.
  • Some are way too small to notice and some that we notice can create little or severe destruction.
    • Earthquakes begin in the lithosphere.
    • They begin within 100 kilometers of the Earth's surface.

Seismic Waves

• Seismic waves are vibrations that travel through the earth at various speeds carrying the energy that is released by an earthquake.
  • They move in the form of ripples carrying the large amount of energy of the earthquake.
    • The energy of the seismic waves is the greatest at the epicenter.
    • The seismic waves carry the energy of the earthquake away from the focus.
    • There are three types of seismic waves:
• P waves
  • Also known as primary waves
  • Compress and expand the surface like an accordion.
  • Move through solid and liquid
  • Arrive first during earthquake
• S waves
  • Also known as secondary waves
  • Arrive second in an earthquake and are very violent
  • Move from side to side as well as up and down
  • Cannot move through liquids

• Surface Waves
  • Make the ground roll like an ocean wave or shake severely

Detecting Seismic Waves

• In order to record seismic waves, scientists use seismographs.
• Siesmographs are of two types
  • Mechanical seismographs which have a drum that vibrates when an eathrquake occurs and a stationary pen causes these vibrations o be recorded on paper
  • Electronic seismographs which convert seismic waves to a signal into a signal which can be recorded

Measuring Earthquakes

• Over 20 different measures are used to rate earthquakes
• The three most common rating systems are
  • The Mercalli scale
    • Developed in the early 1900's
    • Used to rate earthquakes based on their intensity
    • Not considered accurate,
    • Its 12 steps describe how the effects of the earthquake affect people
    • An earthquake can have different ratings because it can cause different amounts of damage at locations
  • The Richter Scale
    • Rates based on the size of the seismic waves
    • Developed in the 1930's
    • Moved from the manual seismograph to the electric seismograph
    • Accurately measures small and close-by earthquakes
  • The Moment Magnitude scale
    • Can accurately measure a small, distant and earthquakes of all sizes
    • Geologists combine the strength of the rock and how much movement occurred on the fault

Locating the Epicenter

• The difference between the arrival of P waves and S waves is indicative of hte distance of the epicenter from the seismograph
  • The farther away an earthquake, the greater the arrival time of P and S waves
• At least 3 circles are drawn with 3 different seismographs as centers
  • The point of intersection of these circles is the epicenter.